Radar is an acronym for Radio Detection and Ranging. The term “radio” refers to the use of radio frequency (RF) waves. The detection and ranging part of the acronym is accomplished by timing a delay between transmission of an RF pulse and its subsequent return. If the time delay is Δt, then the range may be determined by the simple formula:R=cΔt/2where c=3×108 m/s and is the speed of light. The factor of two in the formula accounts for the return trip.
Referring now to FIG. 1, the common radar carrier modulation or pulse train and other radar parameters are shown. The pulse width (PW) is the duration of the radar pulse. The rest time (RT) is the interval between pulses. The pulse repetition time (PRT) is the interval between the start of one pulse and the start of a subsequent pulse. PRT is equal to the sum, PRT=PW+RT. The pulse repetition frequency (PRF) is the number of pulses transmitted per second and is equal to the inverse of PRT. The radio frequency (RF) is the frequency of the carrier wave that is being modulated to form the pulse train.
Military organizations use radar communication systems. Until recently, military radar communication systems enjoyed nearly interference-free communication. In recent years, however, wireless network communications have proliferated. As a result, wireless network signals may interfere with military radar communications. Interference between publicly used wireless networks and military radar systems is undesirable for security reasons.
Based on the disclosures by the military organizations, IEEE has defined the IEEE 802.11h specification, which is incorporated herein by reference. IEEE 802.11h attempts to limit wireless networks and wireless network devices from interfering with radar systems. Support for IEEE 802.11h is required in all IEEE 802.11a compliant access points and client stations to avoid interference with military radar. IEEE 802.11h uses two techniques to reduce radio interference: Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC).
When a device that employs DFS detects other devices on the same radio channel, the device switches to another channel if necessary. Typically, an AP transmits beacons and informs the client stations that the AP uses DFS. When the client stations detect radar on a channel, the client stations notify the AP. Based on this information, the AP uses DFS to select the best channel for network communications that will not interfere with radar.
TPC reduces interference by limiting the transmit power of the network devices to a minimum level that is necessary to reach a farthest client station. Maximum power limits may be set within the AP and are imposed on the client stations that associate with that AP. By limiting the transmit power of client stations, TPC may limit interference with radar.
Once a wireless network device detects radar, the network should stop using that channel within a predetermined time, such as 10 seconds. Communication on that channel may be blocked for a subsequent period of time, such as half-an-hour. Some network devices may falsely detect radar on a channel. For example, a network device may incorrectly conclude that noise such as a signal generated by a microwave appliance or other device is a radar signal. The network will unnecessarily block the channel despite the fact that the detected signal is not a radar signal. As false detections increase, additional channels may be blocked and fewer channels will remain available for network communications. This can significantly degrade network performance.